Typically, licensed wireless systems such as general system for mobile communications (GSM), universal mobile telecommunications system (UMTS), and code division multiple access (CDMA) and unlicensed wireless system such as wireless fidelity (Wi-Fi), Bluetooth, and radio frequency identification (RFID) are deployed individually in overlapping coverage areas. With its relatively high data rate and low maintenance cost, the popularity of unlicensed wireless systems has increased dramatically over recent years. However, the unlicensed wireless systems are limited by having small service area and low mobility. In the meantime, the licensed wireless systems are advantageous by having broad service areas and high mobility, in spite of low data rate and high usage cost.
Recently, with the advent of unlicensed mobile access (UMA) technologies converging the capabilities of licensed and unlicensed wireless technologies, UMA-enabled dual-mode terminals have been developed.
In UMA technology, a UMA network controller (UNC) enables access to voice, data, and other mobile services of a cellular communication system via an access point (AP) of an unlicensed wireless system (i.e., a wireless local area network (WLAN)) without modification of cellular system equipments such as mobile switching center (MSC), serving general packet radio service (GPRS) support node (SGSN), and gateway GPRS support node (GGSN).
Using an UMA-enabled dual-mode terminal, a subscriber can roam between cellular networks and unlicensed wireless access networks such as WLAN without breakage of ongoing voice and data services, whereby the UMA technology distributes traffic load of the cellular networks and thus improves service quality.
In order for a mobile terminal to access the services provided by a licensed wireless system via an unlicensed wireless system, the mobile terminal has to register with an UMA network (UMAN) and establish a logical connection with the UNC (i.e., a UMA radio resource (URR) connection).
Here, the mobile terminal initiates the establishment of the URR connection by sending a URR REQUEST message with activation of a timer (TU3908). If the mobile terminal receives a URR REQUEST ACCEPT message from the UNC in response to a URR REQUEST message before the TU3908 expires, a URR connection is successfully established between the mobile terminal and the UNC.
In contrast, if the mobile terminal receives a URR REQUEST REJECT message or no response message before the TU3908 expires, the URR connection fails between the mobile terminal and the UNC.
The URR REQUEST REJECT message is sent by the UNC when the UNC experiences a temporal resource shortage. Also, the UNC may not receive the URR REQUEST sent by the mobile terminal or the mobile terminal may not receive the URR REQUEST ACCEPT message or the URR REQUEST REJECT message sent by the UNC because of temporal network problems.
Accordingly, there has been a need for a reliable logical URR signaling mechanism between the mobile terminal and the UNC.